From Sean Penn and the Oscars to Keith Olbermann and Bill O’Reilly at their cable TV perches, and with a dash of national security issues for good measure, we’re “all atwitter” — literally.

Twitter — the 140-character social media phenom — is used by just 23 percent of adult Americans who are online, according to a 2014 Pew Research Center report. For teens, surveys say it’s about one-quarter of online regulars, and rising quickly.

But wherever the numbers fall, Twitter has become a free speech and free press touchstone. The once scorned-as-mere-gossip “Twittersphere” often sets the agenda for national chatter, serious discourse, press accountability and the occasional verbal-call-to-arms that once was the province of daily newspapers and, later, network television.

Most recently, ESPN’s oft-controversial commentator Keith Olbermann lost his TV chair for the rest of the week after a series of rants with and about Penn State students and the value of their education — tweets that wandered off into seeming criticism of student fundraising efforts that raised more than $13 million for pediatric cancer research.

Olbermann ultimately posted: “I apologize for the PSU tweets. I was stupid and childish and way less mature than the students there who did such a great fundraising job.” ESPN management wasted no time in reacting, saying management was “aware of the exchange Keith Olbermann had on Twitter last night regarding Penn State. It was completely inappropriate. ... ESPN and Keith have agreed that he will not host his show for the remainder of this week.”

Fox provocateur-extraordinaire Bill O’Reilly used Twitter in a decidedly non-apologetic manner. On the offensive after an article in Mother Jones magazine attacked his claims to have reported from a “war zone” during the 1982 Falklands War, on Feb. 20 @oreillyfactor had this tweet: “Bill has decided to release Friday’s Talking Points Memo EARLY in order to address the Internet guttersnipes.” Two days later: “Bill calls in to Media Buzz with Howard Kurtz to address the left’s latest smear campaign against him.”

One of the most creative — and nonpartisan — Twitter comments on the controversy came from @)lybr3: “I think they should have replaced O’Reilly about 10-12 years ago when he turned into angry Grandpa. Mother Jones is a joke though.”

Actor Sean Penn caused one of the larger Twitter-spasms late Sunday when at the end of a long Academy Awards program, in announcing Best Picture winner Alejandro González Iñárritu, Penn said, “Who gave this son-of-a-bitch a green card?” Penn and others insisted it was dark humor between friends, and a joke aimed at fellow U.S. directors.

Backstage and later online, Iñárritu said he found the remark “... hilarious. Sean and I have that kind of brutal relationship.” But on Twitter, critics said the comment was offensive to immigrants regardless, and many chose to attack Penn by tweets referencing accounts of domestic abuse and violence in public from the actor’s past.

In fact, recent 87th Academy Awards show prompted a larger Twitter campaign — one aimed at the overall lack of diversity in the competition. In January, when nominations were announced, social media guru April Reign created the #OscarsSoWhite, which quickly gained a sizeable following. Reign moderated a live-tweet session during the Feb. 22 telecast, asking Twitter followers to “do anything” other than watch the Oscar show.

For those who did watch, there was another “Twitter” moment. “Citizenfour,” a film about NSA whistleblower Edward Snowden, took the “Best Documentary” award. Host Neil Patrick Harris remarked that “... Snowden could not be here tonight for some ‘treason.’” The pun drew tweets from many who consider the still-on-the-run Snowden to be a national hero for leaking secrets about government surveillance programs. One Twitter post said Harris “is now dead to me.” But @EricBoehm87 posted: “I laughed at the ‘treason’ joke. Snowden’s no traitor, but that was funny. And NPH also made fun of Meryl Streep. Nothing off limits.”

All of this is worth noting because of what we might call the “atmospherics” around the First Amendment’s protection of freedom of speech. None of the tweets noted — even those regarding Snowden — were likely to bring down government’s heavy hand of censorship. But such exchanges by thousands on myriad topics is a new kind of national dialogue, where uncomfortable issues are held up for comment, where public figures are held accountable and where millions can talk to each other in what are online town hall meetings.

A new kind of “marketplace of ideas” to be sure. But a valuable one. Even at a sentence at a time.